FTO or "Flexible" Time Off systems shift the burden to the employee to ensure that they're taking off a healthy amount of time. I have worked under both systems, and always try to ensure that I take at least 4 weeks off per year.
Of course, employees that don't do this may end up working more and taking less time off, as you said. Without the PTO balance giving them a "justification" for subverting perceived work culture, some employees may also feel more pressure to not take time off; or, maybe they just don't think about it. For those reasons, combined with the fact that it allows companies to avoid having a "negative balance" when an employee with accrued time off leaves, makes the argument that FTO systems are at least somewhat predatory and lopsided not too much of a stretch.